Porously coated open-structure substrate and method of manufacture thereof

ABSTRACT

A method for sintering a porous coating on an open-structure substrate, i.e., a substrate with pre-made pores or openings. The open-structure substrate is spread with a coating paste that is prepared with such a viscosity so that the paste will not drip through the pores/openings on the open-structure substrate. The coating paste is then sintered to form a porous layer on the surface of the open-structure substrate. Optionally, the porous coating may be further coated with a catalyst for fuel cell applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of copending application Ser. No. 09/956,432, filedSep. 20, 2001, now U.S. Pat. No. 6,656,526.

TECHNICAL FIELD

The technical field relates to a process for the fabrication of a porouscoating on top of an open-structure substrate. The porously coatedopen-structure substrate is preferably used in fuel cell applications.

BACKGROUND

A fuel cell is an electrochemical apparatus wherein chemical energygenerated from a combination of a fuel with an oxidant is converted toelectrical energy in the presence of a catalyst. The fuel is fed to ananode, which has a negative polarity, and the oxidant is fed to acathode, which, conversely, has a positive polarity. The two electrodesare connected within the fuel cell by an electrolyte to transmit protonsfrom the anode to the cathode.

One of the essential requirements of typical fuel cells is the easyaccess to the catalyst and a large surface area for reaction. Thisrequirement can be satisfied by using an electrode made of anelectrically conductive porous substrate that renders the electrodepermeable to fluid reactants and products in the fuel cell. To increasethe surface area for reaction, the catalyst can also be filled into ordeposited onto a porous substrate.

However, these modifications result in a fragile porous electrode thatneeds additional mechanical support. An alternative is to sinter aporous coating on a solid substrate and then fill or re-coat the porouscoating with a catalyst. The substrate can be made of conductivematerials or patterned with a conductive material. The porous coatingcan be made of non-conductive materials, such as ceramics or silicon, orconductive materials such as carbon, ceramic-metal mixture or metals.Typically, the coating material, in the form of fine powders, is mixedwith a liquid organic “binder” such as glycol to form a coating mixture.The coating mixture is spread on the substrate and is baked in an oven.The binder is burned off and the coating material is sintered on thesubstrate to form a porous layer. A catalyst is deposited in the porouslayer to provide a large catalytic surface. The substrate is then etchedfrom the backside to create openings so that fuel on the cathode sideand oxygen on the anode side can reach the active catalytic surfacesthrough the openings and the porous layer. The etching process, however,is time consuming and requires specially designed machinery.

SUMMARY

A method for sintering an open-structure substrate with a porous coatingis disclosed. The open-structure substrate is a substrate withpre-formed openings (i.e., pre-formed pores, channels, passageways,etc.), which allow liquids and gases to pass from one side of thesubstrate to the other side of the substrate. The method is based on thefact that a viscous solution can remain a continuous layer after beingapplied onto open-structure substrates, such as screens or expandedfoils, without dripping through the openings of the substrate.

Briefly, for a chosen open-structure substrate, a coating paste isprepared by mixing a solid coating material with a liquid binder. Thecoating material and the binder is mixed at a ratio such that theviscosity of the paste is high enough to prevent the paste from drippingthrough the openings on the substrate. The paste is spread on thesurface of the open-structure substrate, and is subjected to a heatingprocess to remove the binder and to sinter the coating material on thesurface of the open-structure substrate to form a porous coating. Sincethe openings on the substrate are pre-formed before the coating process,this method eliminates the expensive and time-consuming etching stepafter the sintering. The method can be used to manufacture electrodesfor fuel cells or any other applications which require a porous coatingon an open-structure substrate.

In an embodiment, the paste comprises fine metal powders, such as zincor silver powders, a viscous binder, such as glycol, and, optionally, aflux. A flux is a reducing agent that serves to remove the oxidizedsurface layer of metal particles.

In another embodiment, the porous coating on the open-structuresubstrate may be further coated with a catalyst.

In yet another embodiment, the paste may be prepared in the form of asolgel. A solgel is prepared by peptizing a coating material, such assilicon oxide or metal oxide, with water or a water-miscible alcohol,such as methanol, ethanol, isopropanol, ethylene glycol and the like, toform a viscous polymeric sol. The viscous polymeric sol is heated at arelatively low temperature (usually less than 100° C.) to form aheat-set gel. The heat-set gel is then heated in the presence of oxygenat a temperature and for a period of time sufficient to oxidize andvolatilize any remaining vapors and organic materials from the gel toform a solid porous product.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description will refer to the following drawings, in whichlike numerals refer to like elements, and in which:

FIG. 1A depicts an open-structure substrate.

FIG. 1B depicts an open-structure substrate covered with a paste.

FIG. 1C depicts an open-structure substrate with sintered porouscoating.

FIG. 1D depicts a porous coated open-structure substrate with optionalcatalyst coating.

DETAILED DESCRIPTION

FIG. 1A shows an open-structure substrate 100 having an upper surface102, a lower surface 104, and a plurality of openings 106. The numberand size of the openings 106 can be variable, so long as they provideenough permeability so that the liquid on one side of the substrate mayreach the other side in sufficient quantities. The substrate 100 may bemade of conductive materials, such as metal, carbon, metal-ceramicmixture, or non-conductive materials, such as glass, ceramic, flexmaterial such as Kapton (Dupont) or Upilex (Ube), for example. Anon-conductive substrate 100 may be patterned with a conductive material(not shown in the figure) for certain electrical applications.

FIG. 1B shows the open-structure substrate 100 covered with a coatingpaste 120. The coating paste 120 comprises a coating material 122 and abinder 124. The coating material 122 may be in the form of fineparticles. The diameters of the particles may vary depending on thedesired porosity of the porous coating. In general, smaller particleslead to smaller pores and a larger surface area, while larger particlesresult in larger pores and a smaller surface area. The coating materialmay be carbon particles, glass beads, silicon powder, ceramic powder,metal particles, or any other material or a mixture of materials thatmay form a porous layer after a sintering process. Nonconductive coatingmaterials, such as silicon or ceramic, may be pre-coated with a thinlayer of conductive material, such as zinc, before sintering so that thefinal coating would be conductive.

Metals that are used for the porous coating are not limited to specifickinds. The following substances may be preferably used: Ni, Cu, Al, Fe,Zn, In, Ti, Pb, V, Cr, Co, Sn, Au, Sb, Ca, Mo, Rh, Mn, B, Si, Ge, Se,La, Ga and Ir. Each metal listed above may be used in the form of oxideand sulfide thereof and a simple substance or a mixture, includingcompounds of these metals. The metals may be used in a powder form toincrease the surface area. The peripheral surfaces of the metal powdersare desired not to be convex or concave so that they do not intertwineone another. Thus, the metal powders are preferably spherical,dice-shaped, square piller and columner.

The temperature for sintering a metal powder should be high enough topartially melt the metal particles in order to form a sintered porousmetal layer. However, overheating may completely melt the metalparticles and destroy the porosity of the sintered layer. To prevent theoverheating, metal particles (hereafter defined as “core particles”) maybe pre-coated with a thin layer of a cover metal with a lower meltingtemperature. For example, copper core particles may be coated with acover layer of zinc. The coated core particles are then sintered at themelting temperature of the cover metal. Since the melting temperature ofthe cover metal is lower than the melting temperature of the core metal,the core particles will not melt and will maintain the porosity of thelayer after the sintering process. A similar method may also be used forsintering non-conductive coating materials (hereafter defined as“non-conductive core particles”). In this case, the non-conductive coreparticles may be pre-coated with a layer of cover material having amelting temperature lower than that of the core particles, and sinteredat the melting temperature of the cover material.

The binder 124 may be glycol, wax, a solvent, or any other viscousliquid that is evaporable during the sintering process.

The binder 124 and the coating material 122 are mixed at a ratio thatresults in a paste 120 with a viscosity high enough to prevent the paste120 from dripping through the openings 106 on the substrate 100.

The paste 120 is then applied to the upper surface 102 of theopen-structure substrate 100 to form a pre-sintering coating. If theopen-structure substrate 100 is patterned with a conductive layer, thepaste 120 may be screen printed on the open-structure substrate 100 sothat the location and shape of the sintered porous coating can conformto the patterned conductive layer.

If the coating material is metal particles, a reducing agent, called aflux, may be added to the paste to remove oxidized surface layer of themetal particles so that the particles can melt to each other during thesintering process. The choice of flux is dependent on the type of metalthat needs the treatment.

The next step is to bake the paste 120 in an oven to dry out the binder124. As shown in FIG. 1C, the coating material 122 is sintered and formsa porous coating 130 on the surface of the open-structure substrate 100after the baking process.

The baking conditions, i.e., baking time and temperature, may varydepending on the coating material 122 and the binder 124. For metalliccoating materials, the paste 120 is preferably baked under a conditionthat partially melts the metal particles in order to form a layer ofporous metal.

FIG. 1D shows an optional step of further depositing a layer ofcatalytic coating 140 on the porous coating 130. Referring to thecomposition of catalytic coating for fuel cells using methanol,catalytic materials such as Pt—Ru and Pt—Ru—Os, are found to beeffective in converting methanol to protons without poisoning other fuelcell constituents. If the porous coating 130 is made of conductivematerials, the catalyst may be deposited onto the porous coating 130 byelectroplating, electroless plating, atomic layer deposition, or anyother process that is capable of coating the surface of a conductiveporous layer. In this case, the porous coating 130 may function as acurrent collector. If the porous coating 130 is made of non-conductivematerials, a conductive material may be deposited onto the surface ofthe non-conductive porous coating 130 by electroless plating, atomiclayer deposition, or any other process that is capable of coating thesurface of a non-conductive porous layer. The catalyst 140 is thendeposited onto the conductive material by electroplating, electrolessplating, atomic layer deposition, or any other process that is capableof coating the surface of a conductive porous layer. Alternatively, thenon-conductive porous coating 130 may be directly coated with thecatalyst 140 by atomic layer deposition, or any other process that iscapable of coating the surface of a nonconductive porous layer.

Although preferred embodiments and their advantages have been describedin detail, various changes, substitutions and alterations can be madeherein without departing from the spirit and scope of the sinteringprocess as defined by the appended claims and their equivalents.

What is claimed is:
 1. A method for sintering a porous coating on anopen-structure substrate comprising the steps of: selecting anopen-structure substrate with openings of desired sizes, saidopen-structure substrate having a first side and a second side; admixinga coating material with a binder to form a paste having a viscosity highenough to prevent said paste from dripping through the openings on theopen-structure substrate; applying the paste to the first side of saidopen-structure substrate to form a paste layer of a desired thickness;and heating the paste layer at a sintering temperature for a period oftime to evaporate said binder and sinter said coating material to saidopen-structure substrate to form a porous coating on the open-structuresubstrate.
 2. The method of claim 1, wherein the open-structuresubstrate is selected from the group consisting of carbon, ceramic,glass, plastic film, and metal sheets.
 3. The method of claim 1, whereinthe coating material is conductive.
 4. The method of claim 3, furthercomprising the step of depositing a catalytic coating on top of theporous coating, wherein said catalytic coating has a differentcomposition than said porous coating.
 5. The method of claim 4, whereinthe catalyst is deposited by electroplating, electroless plating, oratomic layer deposition.
 6. The method of claim 3, wherein the coatingmaterial comprises metal particles.
 7. The method of claim 6, furthercomprising the step of admixing the coating materials with a flux. 8.The method of claim 1, wherein the coating material is non-conductive.9. The method of claim 1, wherein the coating material comprises a coreparticle coated with a cover layer, wherein the core particle and thecover layer each has a melting temperature, and wherein the meltingtemperature of the cover layer is lower than the melting temperature ofthe core particle.
 10. The method of claim 1, wherein the coatingmaterial comprises ceramics.
 11. The method of claim 1, wherein thecoating material comprises silicon.
 12. The method of claim 1, whereinthe binder material is selected from the group consisting of glycol andwax.
 13. The method of claim 1, further comprising the step ofdepositing a catalytic coating on top of the porous coating, whereinsaid catalytic coating has a different composition than said porouscoating.
 14. The method of claim 13, wherein the catalytic coating isselected from the group consisting of Pt, Pt—Ru and Pt—Ru—Os.
 15. Themethod of claim 1, wherein the paste is prepared in the form of asolgel.
 16. A porously coated open-structure substrate manufactured bythe method of claim
 1. 17. A method for sintering a porous coating on anopen-structure substrate comprising the steps of: selecting an openstructure substrate with openings of desired sizes; said substratehaving a first side and a second side; admixing a coating material witha binder to form a paste having a viscosity high enough to prevent saidpaste from dripping through the openings on the substrate; applying thepaste to the first side of said substrate to form a paste layer of adesired thickness; and heating the paste layer at a sinteringtemperature for a period of time to evaporate said binder and sintersaid coating material to said open-structure substrate to form a porouscoating on the substrate; wherein the substrate is selected from thegroup consisting of metal, carbon, metal-ceramic mixture, glass,ceramic, and plastic film; the coating material is selected from thegroup consisting of metal powder, carbon, silicon, and ceramic; thebinder is selected from the group consisting of glycol and wax.
 18. Aporously coated open-structure substrate manufactured by the method ofclaim
 17. 19. The method of claim 1, wherein said open-structuresubstrate is liquid permeable.
 20. The method of claim 17, wherein saidopen-structure substrate is liquid permeable.
 21. The method of claim 1,wherein said porous coating is non-catalytic.
 22. The method of claim16, wherein said porous coating is non-catalytic.